96 BIOLOGICAL CHEMISTRY 



possible to show that in general there is an oxidase associated 

 with an oxidisable substance. 



White flowers are due to the absence of either the oxidase or 

 oxidisable substance. There are thus two possible forms of 

 whiteness, and these can be shown to exist. Some white 

 flowers become coloured when treated with an oxidase : that 

 is, they contain oxidisable substance but no oxidase. Other 

 white flowers do not become coloured when treated with an 

 oxidase, that is, they do not contain the oxidisable substance, 

 but an oxidase can be extracted from them. 



Turning now to the chemical nature of the plant pigments 

 we find that they are derivatives of cyclic compounds combined 

 with sugars to form glucosides called anthocyanins. 



Anthocyanins are hydrolysed by an enzyme into sugar 

 and anthocyanidine. The anthocyanidine is a reduction 

 product of flavonol, and it can exist as colourless or coloured 

 tautomeric modifications.* 



Animal pigments may serve the purpose of concealment. 

 Thus we find animals with pigmentation resembling their 

 background. Some- of these pigments are possibly derived 

 - from the plants on which they feed, but in other cases the 

 pigments are contained in special cells called chromatophores. 

 The chromatophores by movements of expansion or contrac- 

 tion can alter the colour of the surface so that the animal 

 can change its colour, as in the case of the chameleon, fish, 

 amphibia, etc. 



We find, however, certain animal pigments which are of 

 special importance. The black pigment which lines the eye 

 is required for optical purposes, so that light can neither 

 diffuse through the wall of the eye, nor is it reflected inside 

 the eye. The black pigment of the negro's skin is a protection 

 to the deeper layers. The sunlight is absorbed on the surface, 

 and the heat produced got rid of by evaporation of sweat, 

 but if the rays penetrated to the subcutaneous tissues, where 

 arrangements for heat loss are less efficient, overheating and 

 destruction would occur, followed by toxaemic symp- 

 toms. 



These two black pigments are composed of melanin, which 

 is a black insoluble substance derived from tyrosin by the 

 action of an oxidising enzyme tyrosinase. 



The browning of the human skin when exposed to the 

 sun and the increase in pigmentation in some diseases is not 

 yet understood. 



* O. Rosenheim, Biochem. Journ., 1920, vol. 14, p. 178. 



